Novel hierarchical SnO/BiOX (X = Cl, Br, I) p-n heterostructures with enhanced photocatalytic activity under simulated solar light irradiation.

SnO/BiOX (X = Cl, Br, I), a series of p-n-heterojunction-based photocatalysts, were prepared by a combination of an ultrasonic-assisted precipitation-deposition method and hydrothermal method. The photodegradation of Rhodamine B among all the materials, under simulated solar light irradiation, was investigated in detail. The photocatalyst test showed that the novel composite, SnO/BiOCl, was able to degrade 99% of Rhodamine B (RhB) and its intermediates in 9 minutes, which is faster than SnO/BiOBr (21 min) and SnO/BiOI (12 min). Moreover, the degradation rate of RhB for SnO/BiOCl samples (Sn : Bi = 1/4) was the highest, about 99%, slightly higher than that of SnO/BiOCl-1/8 (95%), which was significantly higher than those of SnO/BiOCl-1/2 (78%), BiOCl (77%), P25 (62%) and SnO (16%) after nine minutes of irradiation under a xenon lamp. It can be inferred that when the bismuth/tin ratio was optimum, BiOCl enabled the formation of the enough space charge regions on the surface of SnO, which promoted the separation of photogenerated electron-hole pairs. This implied that high-quality interfaces in the heterostructure catalysts play a key role in improving the photocatalytic performance. The enhanced photocatalytic performance can be attributed to the synergistic effects from two main factors: (1) the layered multi-stage structure increases the scattering of light on the catalyst surface, which proves to be beneficial in enhancing the absorption of the visible light; (2) the p-n heterojunctions between SnO and BiOX (X = Cl, Br, I) efficiently promote the separation of photogenerated carriers and accelerate the migration of photogenerated carriers. In addition, the results of the 'active species trapping' experiment illustrated that in the SnO/BiOCl composite, holes contribute more to the high photocatalytic performance, while hydroxyl radicals show less importance to degrade RhB. Moreover, the photocatalytic mechanism was also discussed based on the investigation of reactive species and the band structure of SnO/BiOCl.